Dispenser containers for viscous fluids are well known. For example, a dispenser for viscous cosmetics such as toothpaste or lotion is described in Kolaczinski et al., U.S. Pat. No. 4,154,371. The Kolaczinski et al. container comprises a piston forming the bottom of the container and being displaceable in the container; a compressible container part, lying in front of the piston; a mouthpiece for withdrawal of the contents, of flexible and elastic material; and a delivery valve opening on excess pressure in the container. The piston is secured by a blocking pawl against any displacement causing enlargement of the interior space of the container.
A similarly useful dispenser is also described in Spatz, U.S. Pat. No. 3,361,305. The piston of the dispenser described, which piston effectively pushes out the container filling, does not require a locking means, for example, catches or ratches, against displacement due to internal displacement or pressure in the container since, when the diaphragm pump is operated, either its inlet valve or its outlet valve is always closed and the material is introduced into the pump chamber by vacuum.
Proper operation of the Spatz dispenser, especially to achieve a sufficiently high filling level of the diaphragm pump, requires freely moving and tightly closing valves. Several flap valves are arranged in the top surface of the container between the storage chamber of the container and the pump chamber, which flap valves are arranged substantially symmetrically to the center of the top surface. While the flap valve openings consist of holes whose surface is slightly inclined to the inside from the edge to the center of the top surface, the respective flaps are a part of a plate arranged in the pump chamber on the top surface. When the diaphragm pump of the dispenser is operated, a flexible, elastic surface is moved downward, perpendicularly to the top surface containing the flap valves in such a way that the product contained in the pump chamber is ejected or dispensed through the delivery valve of the dispenser. When the flexible, elastic surface returns to its normal, unstressed position, the delivery valve is closed because of the resulting vacuum and the inlet flap valves arranged in the top surface open in such a way that new material is drawn into the pump chamber from the interior of the container, that is, from the storage chamber, by the pressure differential, or vacuum, produced in the pump chamber.
Proper operation also requires that as the piston slidingly advances along the cylindrical inner wall of the container, that there be a tight seal, for example, by means of packing washers. Leakage at the points of contact between the piston and inner wall can result in air entering the interior of the container between the piston and the top surface and being delivered instead of viscous product when the diaphragm pump is operated. As in the case of leaks in the inlet and outlet valves, such leaks around the piston have an adverse effect on the filling level of the pump and thus on the dosing accuracy.
When the interior of the container is filled with viscous product to be delivered later by means of the diaphragm pump, the leaving of some air in the container can usually not be avoided. Such air is pushed ahead of the product introduced into the container and thus into the pump chamber. Also, air pushed ahead of the product accumulates in the head of the dispenser when the latter is closed, as is typical, with a lid to prevent drying of the product. Air captured in the interior of the container when the piston is inserted can not escape, since the piston, as mentioned above, must bear tightly on the inner cylinder wall if it is to perform its function properly. Without special measures which interfere with the automatic manufacture and filling of the dispenser, considerable inconveniences may appear, particularly at the start of the operation of the diaphragm pump, because only air is delivered substantially at first.